For fun, I built a guitar from a 2x4, some scrap trim, and an old license plate. I've entered this 'ible in the 2x4 Contest. If you like what you read here, please vote for me.
Step 1: Cutting to Size
I measured off how long I needed the neck piece to be, then marked it on the 2x4. (I forgot to take a photo of the raw 2x4 prior to cutting...sorry) I roughly cut out for the neck shape and head stock. Then I cut some scrap pieces to make a box for the guitar body. I made the box a little larger than 6" x 12" so the edges of the plate wouldn't hang over. They can be a little rough around the edges, so I wanted a little extra protection.
Step 2: Shaping the Neck
Here is the 2x4 after the neck and headstock cuts. I drilled three holes through the head (sideways) for the eye bolts (tuners) which will be added later.
Next, I cut out a little from the tope part of the 2x4 that will be inside the box. This way the license plate will have room to vibrate. Later, I'll add a piece to fit between the 2x4 and the plate right under the bridge for support. If you want to add a magnetic pickup, this would be a good time to make the appropriate cuts to fit it in.
I next drilled out the center of the headstock so I could attach the strings to the eye bolt tuners. The downward angle of the headstock will allow plenty of break angle for the strings to prevent buzzing.
Pine isn't typically use for necks as the horizontal grain that you usually find in pine lumber doesn't have the necessary strength to resist the string tension. I took two steps to prevent that. One, I turned the 2x4 sideways so that the grain runs vertical, improving the strength of the neck. two, I glued a 1/4" x 1.5" strip of oak to the top face. This is your fretboard. I'm planning on keeping this one fretless, but if you want to fret yours, go right ahead. It's all personal choice.
Step 3: Building the Body
I glued/nailed together some scrap pieces to create the body for this guitar. As license plate edges can be sharp, I made it a little oversize to help prevent contact with those edges. I glued in four scrap pieces along the inside edges as the holes in the plate don't line up over the sides.
Step 4: Attaching Neck to Body
I cut a half-lap joint to allow the neck to fit over the front wall of the box. Once glued in, it will be plenty strong. The end of the 2x4 fits against the back wall and a couple of screws will be driven in from the outside to secure it.
Step 5: Attach License Plate to Box
Attach license plate to box with screws and washers. I chose "finish washers" as I prefer that look. A piece of thin plywood was cut to size and screwed onto the back. I didn't glue the plate or the plywood as that will allow easy access to the inside if I choose to add electronics later.
Step 6: Tailpiece, Tuners, and Strings
I used an old hinge for a tailpiece to anchor the strings.
I inserted eyebolts through the headstock, then put a washer and a wing nut on the other side. I'll use these for tuners.
I've used a couple of different sizes of threaded rod for the bridge and nut. String pressure will hold the bridge in place, but I made a shallow groove in the neck to help the nut stay put.
For this build, I used the lowest three strings (E/A/D) out of a pack of acoustic guitar strings. They'll be tuned to D/A/D. Run the strings through the three holes in the hinge until the ball end catches. If the hole is too large to catch the end, loop the string through the hinge and back through the hole in the ball end. Stretch the strings over the bridge and nut, and knot them over the tuners, then tighten the tuners until the strings reach pitch. Electronic tuning devices can be purchased fairly inexpensively online. The strings will have to be retuned often for several days until they settle.
Step 7: Finally Done...and Other Options
Now we're done. Here's what mine looks like.
There is a lot of customization that can be done in this build. I'll go over a few options here, but there are many sites which have LOTS of information for the DIY instrument maker. Cigar Box Nation and Handmade Music Clubhouse are two good ones.
Nut/bridge material: these can be anything hard...hardwood, bolts, bone, etc. Anything to get the vibration to the license plate will work for a bridge.
If you're playing solely acoustically, then a sound hole will be needed. I'd suggest putting one in the top wall of the box, facing you as you play. It could go in the license plate, but I personally prefer the look of a solid plate.
Electronics: Many folks like to amplify their creations. You could mount a piezo disc under the plate and run it to a jack. Or add a magnetic pickup and cut out a hole in the plate for it to fit. Or mount a magnetic inside the box and let it pick up the plate's vibrations. Lot's of possibilities.
Frets: While I built this one fretless, you could easily add frets. Many online sites (google "fretfind2d" for one) calculate the fret locations, given the scale length (nut to bridge...I went with 25".) I plan to locate certain frets (3, 5, 7, 12) and mark them along the top of the neck for reference as this will be primarily a slide guitar.
Most importantly, have fun playing your creation! I'll make a little video after the strings settle so you can hear the sound. If you make one, please attach a short video sharing your sound too.